Liquid crystal displays are commonly used in portable computer systems, televisions, and other electronic display devices. Most of the large area, high performance LCDs require a source of light for operation. Backlighting the LCD has become the most popular source of light in LCD devices.
Backlighting of typical brightness uses scattering to transmit light. For a very high brightness backlighting system, typically a serpentine fluorescent lamp or a flat fluorescent lamp is used. Each of these light sources emits light in all directions and is not focused toward the display. As a result, the energy efficiency is low. Furthermore, a great deal of power is required which creates a power dissipation problem. Moreover, these prior technologies are not suitable for a stacked configuration and are incapable of illuminating a given active area of a display with light of different spectra efficiently.
The technologies described in U.S. Pat. No. 5,359,691 and U.S. Pat. No. 5,390,276, entitled "Backlighting System with a Multi-Reflection Light Injection System and Using Microprisms" and "Backlighting Assembly Utilizing Microprisms and Especially Suitable for Use with a Liquid Crystal Display," issued Oct. 25, 1994 and Feb. 14, 1995, respectively, to Tai, et al., provides very efficient collimated backlighting. However, each technology includes a single light pipe, and thus operates in one mode. Therefore, with one light source, a single light pipe's light output is limited to a single spectra. Accordingly, the previous technology is not suitable for backlighting displays for outdoor applications or for use in conjunction with night vision goggles. Moreover, the dimming ratio of a single light pipe configuration is low.
A multi-mode operation backlighting system is therefore desirable. Moreover, it would also be advantageous to have a high dimming ratio as well as the capability of spectral variation along with a high energy efficiency.